The field of art to which this invention pertains is the hydrocracking of a hdrocarbonaceous feedstock. Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline by hydrocracking a hydrocarbon feedstock derived from crude oil, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 700xc2x0 F., usually at least about 50% by weight boiling above 700xc2x0 F. A typical vacuum gas oil normally has a boiling point range between about 600xc2x0 F. and about 1050xc2x0 F.
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs, higher liquid product yields and desired product streams. In one case of hydrocracking the preferred product is a middle distillate while achieving maximum quantities of diesel boiling range hydrocarbons. However, in some cases and in some parts of the world it is highly desirable to produce maximum quantities of middle distillate including diesel and selective amounts of propane and butane for use as liquefied petroleum gas (LPG). Traditionally, this scenario would suggest the use of two independent processes to provide these two desirable product streams. However, the present invention enables the integration of hydrocracking processes in order to produce both middle distillate and LPG which process greatly improves the efficiency and economic benefits of producing both desirable product streams. It was previously believed that the production of two product streams having such widely diverse boiling ranges could not advantageously be produced in a single processing unit.
U.S. Pat. No. 5,720,872 discloses a process for hydroprocessing liquid feedstocks in two or more hydroprocessing stages which are in separate reaction vessels and wherein each reaction stage contains a bed of hydroprocessing catalyst. The liquid product from the first reaction stage is sent to a low pressure stripping stage and stripped of hydrogen sulfide, ammonia and other dissolved gases. The stripped product stream is then sent to the next downstream reaction stage, the product from which is also stripped of dissolved gases and sent to the next downstream reaction stage until the last reaction stage, the liquid product of which is stripped of dissolved gases and collected or passed on for further processing. The flow of treat gas is in a direction opposite the direction in which the reaction stages are staged for the flow of liquid. Each stripping stage is a separate stage, but all stages are contained in the same stripper vessel.
International Publication No. WO 97/38066 (PCT/US 97/04270) discloses a process for reverse staging in hydroprocessing reactor systems.
U.S. Pat. No. 3,328,290 (Hengstebeck) discloses a two-stage process for the hydrocracking of hydrocarbons in which the feed is pretreated in the first stage.
U.S. Pat. No. 5,114,562 (Haun et al) discloses a process wherein distillable petroleum streams are hydrotreated to produce a low sulfur and low aromatic product utilizing two reaction zones in series. The effluent of the first reaction zone is purged of hydrogen sulfide by hydrogen stripping and then reheated by indirect heat exchange. The second reaction zone employs a sulfur-sensitive noble metal hydrogenation catalyst.
U.S. Pat. No. 5,980,729 (Kalnes et al) discloses. a hydrocracking process which utilizes a hot, high-pressure stripper.
U.S. Pat. No. 4,194,964 (Chen et al) discloses a process for the simultaneous distillation and hydrocracking of hydrocarbon feeds in a single vessel.
The present invention is a catalytic hydrocracking process which provides for the simultaneous production of LPG and distillate hydrocarbons. The process provides higher liquid product yields of middle distillate hydrocarbons while being able to produce a LPG product stream. At least a portion of the hydrocracked naphtha produced in the primary hydrocracking reactor is recycled to a LPG selective second hydrocracking stage. The process of the present invention provides the yield advantages associated with a low conversion per pass maximum middle distillate operation without compromising unit economics. Other benefits of a low conversion per pass operation include the elimination of the need for interbed hydrogen quench and the minimization of the fresh feed preheat since the higher flow rate of recycle liquid will provide additional process heat to initiate the catalytic reaction and an additional heat sink to absorb the heat of reaction. An overall reduction in fuel gas and hydrogen consumption may also be obtained. While enjoying all of these benefits, the process of the present invention is capable of economically producing LPG as defined hereinabove.
In one embodiment, the present invention relates to a hydrocracking process for the simultaneous production of LPG and distillate hydrocarbons which process comprises the following steps: (a) passing a hydrocarbonaceous feedstock and hydrogen to a denitrification and desulfurization reaction zone at reaction zone conditions including a temperature from about 400xc2x0 F. to about 900xc2x0 F., a pressure from about 500 psig to about 2500 psig with a catalyst; and recovering a denitrification and desulfurization reaction zone effluent therefrom; (b) passing the denitrification and desulfurization reaction zone effluent directly to a hot, high pressure stripper utilizing a hot, hydrogen-rich stripping gas to produce a first vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia, and a first liquid stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock; (c) passing at least a portion of the first liquid stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock to a first hydrocracking zone containing a first hydrocracking catalyst and operating at a temperature of about 400xc2x0 F. to about 900xc2x0 F. and a pressure from about 500 psig to about 2500 psig, and recovering a hydrocracking zone effluent therefrom; (d) passing the hydrocracking zone effluent in step (c) to the denitrification and desulfurization reaction zone; (e) condensing at least a portion of the first vapor stream recovered in step (b) to produce a second liquid stream comprising hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock and a second vapor stream comprising hydrogen and hydrogen sulfide in a high pressure separator; (f) recycling at least a portion of the second vapor stream to the first hydrocracking zone; (g) reacting at least a portion of the second liquid stream in a second hydrocracking zone containing a second hydrocracking catalyst to produce LPG boiling range hydrocarbonaceous compounds; (h) passing the resulting effluent from the second hydrocracking zone to the high pressure separator of step (e); and (i) separating at least another portion of the second liquid stream to produce a stream comprising LPG boiling range hydrocarbonaceous compounds and a stream comprising distillate hydrocarbons.
Other embodiments of the present invention encompass further details such as types and descriptions of feedstocks, hydrocracking catalysts and preferred operating conditions including temperatures and pressures, all of which are hereinafter disclosed in the following discussion of each of these facets of the invention.